Changes in the temperature of crystallization, Tcrys, due to mechanical pretreatment were entirely opposite between titania and zirconia hydrogels. The Tcrys of the titania decreased by 18 and 44 K on rotation- and vibration-milling, respectively, while those for the zirconia increased by 14 and 18 K or 16 and 45 K, depending on the starting aqueous solutions, i.e., ZrOCl2 or ZrO(NO3)2, respectively. These phenomena were discussed in terms of the microscopic phase transformation processes in noncrystalline gels during mechanical treatment. The difference between two hydrogels was concluded to be attributed to the differences in the geometry of the construction units, TiO6 and ZrO8, and the bond nature of metal-oxygen and metal-OH between TiO2 and ZrO2.
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